Earth's inner core slows down: New research unveils 70-year cycle

New research has confirmed that Earth's inner core, a solid metal ball spinning independently within the planet, has significantly slowed down. Published in the journal Nature, these findings reveal a 70-year cycle in the core's rotational speed, providing critical insights into the complex dynamics of Earth's interior.

Challenges of Studying the Inner Core
Studying Earth's deep interior directly is impossible. Seismologists gather information by examining the behavior of seismic waves from large earthquakes that reach the inner core. Differences in these waves, observed at different times, allow scientists to measure changes in the core's position and calculate its spin.


"Differential rotation of the inner core was proposed as a phenomenon in the 1970s and '80s," said Dr. Lauren Waszek, a senior lecturer at James Cook University. Seismological evidence published in the 1990s confirmed this rotation. However, subsequent studies have disagreed on the rate and direction of the core's rotation relative to the mantle. Some analyses even suggested the core didn't rotate at all.

Recent Findings
A model proposed in 2023 suggested that the inner core had previously spun faster than Earth but had slowed down significantly. A new study supports this hypothesis, confirming that the core's deceleration is part of a decades-long pattern of speed changes. The research also indicates that changes in rotational speed follow a 70-year cycle.

Cycle of Changes
The new findings indicate that changes in rotational speed follow a 70-year cycle, said study co-author Dr. John Vidale from the University of Southern California. "We've been arguing about this for 20 years, and I think this nails it," Dr. Vidale stated. "I think we've ended the debate on whether the inner core moves and what's been its pattern for the last couple of decades."

Ongoing Debates
Despite these findings, not all scientists are convinced. The implications of the inner core's slowdown for our planet remain an open question. Some experts suggest that Earth's magnetic field could be affected by these changes.

Core and Magnetic Field
The inner core, buried about 3,220 miles (5,180 kilometers) deep, is composed mostly of iron and nickel and is as hot as the surface of the sun. Earth's magnetic field, generated by the movement of metal-rich fluid in the outer core, pulls on this solid ball of metal, causing it to spin. While the core's direct influence on the magnetic field is not fully understood, a slower-spinning core could potentially impact it and slightly shorten the length of a day.

Seismic Observations
Dr. Vidale and his coauthors observed seismic waves produced by earthquakes in the same locations at different times for this study. They found 121 examples of such earthquakes between 1991 and 2023 in the South Sandwich Islands. They also examined core-penetrating shock waves from Soviet nuclear tests conducted between 1971 and 1974. The research revealed that the core is now spinning much slower and accelerates at different rates, "which is going to need an explanation," Dr. Vidale said.

Impact on Earth's Rotation
The depth and inaccessibility of the inner core mean that uncertainties remain. However, changes in the core's spin can be tracked and measured. Researchers highlighted that when the core spins slowly, the mantle speeds up. This shift causes Earth to move faster and shortens the length of a day. "In terms of that effect in a person's lifetime? I can't imagine it means much," Dr. Vidale added.

Despite the challenges in studying the inner core, recent research has provided valuable insights into its behavior. As scientists continue to investigate, they aim to further unravel the mysteries of Earth's inner core and its influence on our planet.